It has been known that a semiconductor device has a vertical type MOSFET with a trench gate structure as a structure in which a channel density is made larger for making a larger current flow through a channel. In this trench gate structure, there is a structure in which an insulation film at the lower side of a gate insulation film inside the channel where a gate electrode is arranged, in other words, an insulation film is made to be thicker at the bottom part of the trench (hereinafter referred to as a bottom part insulation film). (For example, see Patent Literature 1). With this kind of structure, it is possible to decrease a parasitic capacitance Cgd through a shield effect due to having the bottom part insulation film, and also is possible to achieve high-speed switching as compared to a MOSFET without the bottom part insulation film.
The trench gate structure in the vertical type MOSFET having the above-mentioned bottom part insulation film is formed as described in the following. Firstly, subsequent to the formation of a p-type base region on an n-type drift layer, an n+ type source region is further formed by, for example, ion injection into the surface layer part of the p-type base region. Subsequently, a channel is formed from the n+ type source region and penetrates through the p-type base region and reaches the n-type drift layer. Then, the insulation material, which has the same properties as the gate insulation film, is deposited in order to configure the bottom part insulation film so as to bury the channel. Then, the insulation material for burying the channel is etched back to configure the bottom part insulation film. After that, subsequent to the formation of the gate insulation film on the surfaces of the trench and the bottom part insulation film, the gate electrode is arranged on the gate insulation film. Thus, the trench gate structure with a vertical type MOSFET having the bottom part insulation film is formed.
When the bottom part insulation film is formed by this kind of method, the etch-back controllability of the insulation material largely affects the performance of the MOSFET.
In particular, as illustrated in FIG. 5A and FIG. 5B, in a structure where an n-type drift layer J1, a p-type base region J2 and an n+ type source region J3 are arranged, a trench J4 is formed so as to penetrate the p-type base region J2 from the n+ source region J3 and reach the n type drift layer J1. Then a bottom part insulation film J5 is formed so as to leave a space at the bottom part of the trench J4.
In this situation, when the etch-back amount is lower, for example, the upper surface of the bottom part insulation film J5 is located at a position of the trench J4 which is shallower than the bottom part of the p-type base region J2, as illustrated in FIG. 5A. Accordingly, when a gate insulation film J6 and a gate electrode J7 are formed on the bottom part insulation film J5, the gate electrode J7 is not opposite to an entire region of the p-type base region J2 in a thickness direction. Accordingly, a channel region cannot be formed at the entire region of the p-type base region J2 located at the side surface of the trench J4, and hence the vertical type MOSFET with better properties cannot be attained.
On the other hands, when the etch-back amount is larger, for example, the bottom part insulation film J5 becomes slimmer as illustrated in FIG. 5B, and the electric field concentration occurs inside the gate insulation film J6 formed on the bottom part insulation film J5, and the insulation breakdown on the gate insulation film J6 might occur. Accordingly, the long-term reliability of the vertical MOSFET becomes lower.
Thus, with regard to the vertical type MOSFET with the trench gate structure having the bottom part insulation film, there are some difficulties in requesting the etch-back controllability at the formation of the bottom part insulation film and having a narrower process window.